HVAC systems are widely known. “Air conditioning” is a general term for a process that maintains comfort conditions in a defined area. Air conditioning includes sensible heating of the air (referred to generally as heating), sensible cooling and/or dehumidifying of the air (referred to generally as air conditioning, which can be abbreviated as A/C), humidifying the air, and cleaning or filtering the air. HVAC or air conditioning, as used herein, also includes refrigeration systems (e.g., coolers and freezers of consumer, commercial and industrial scale). Therefore, in this description, HVAC can encompass and describe any heating, ventilation, air conditioning, or refrigeration process or equipment. Also, an “A/C unit” can refer to conventional air conditioning unit, a refrigeration unit, or a heat pump.
All conventional A/C or refrigeration systems share the same basic components: a compressor, a condenser coil, a metering device, and an evaporator coil. Compressors compress the gaseous refrigerant and turn it into a subcooled liquid. Condenser coils to allow the refrigerant dissipate heat and become a sub cooled liquid. Metering devices control the flow of the sub cooled refrigerant into the evaporator coil. Evaporator coils expose the refrigerant to the system load turning the refrigerant into a superheated gas. Common metering devices are capillary tubes and in new systems Thermostatic Expansion Valves (TXVs).
The study of air and its properties is called psychrometrics. Typical psychrometric units of measure are dry bulb temperature, wet bulb temperature, relative humidity and enthalpy. HVAC technicians study psychrometrics to accurately predict the final properties of the conditioned air and also to determine if the conditioning equipment is performing the way it was engineered to operate. Air has mass and weight and can therefore store heat energy. The amount of heat that the air can store is dependent upon the conditions of the air. By determining the mass flow rate and understanding the properties of the air and rules of psychrometrics, the amount of heat added or removed from the air by the conditioning device can be quantified. Understanding system airflow is critical to understanding system performance.
The phase change of a refrigerant (from liquid to gas and back to liquid) in a closed system is what allows the refrigerant to transfer thermal energy. To determine the phase state and energy carrying capacity of a refrigerant at any point in the system both the refrigerant pressure and temperature must be known. Pressure gauges are typically used to measure refrigerant pressures and contact thermometers are used to measure refrigerant line (tube) temperatures to infer the refrigerant temperature.
Measurements as typically taken by technicians on their own mean little without knowledge of the design operation. All manufacturers of quality listed equipment have their systems tested and efficiency verified to Air-Conditioning, Heating, and Refrigeration Institute (AHRI) standards. Other independent testing laboratory standards could also be used for testing and efficiency verification. Units having an energy guide label have been tested, and their efficiency can only be guaranteed if the components are matched, the system refrigerant charge is correct, the airflow is correctly set, and the system is installed per the manufacturer's instructions including proper sizing of the equipment.
To achieve the desired efficiency, all manufacturers design their equipment to operate at its rated capacity at one set of conditions at its peak performance. These conditions are known as the AHRI Standard Conditions and are as follows:                Indoor air=80° F.        Relative Humidity=50%        Outdoor air=95° F.        
All equipment listed in the AHRI directory operates at rated capacity under the AHRI standard conditions. Because the AHRI standard conditions are at the high end of the normal range for human comfort, Standard Operating Conditions, or common operating conditions have been established as design conditions for the equipment in the field.                Indoor air=75° F.        Relative Humidity=50%        Outdoor air=95° F.        
Under these conditions the equipment can have a slightly lower operating capacity, and the equipment will operate with different operating characteristics. Along with the standard operating conditions, conditions for airflow and coil temperatures and operating range have also been established. Most if not all manufacturers design these grades of equipment for a nominal 400 CFM airflow per ton for A/C cooling, and 450 CFM/ton for heat pumps.
Government standard tests determine the energy efficiency rating of residential HVAC equipment (cite CFR). This rating is known as Seasonal Energy Efficiency Ratio, or SEER. Higher SEER ratings mean more efficient equipment. The following Tables illustrate some characteristics and nominal operating ranges for air conditioning equipment of these standard grades in certain design operating conditions:
<10 SEER Equipment (R-22 Refrigerant)System Characteristics:Nominal Operating Parameters:Standard size evaporator.Evaporator designed to be 35° F. colder Standard size condenser.than return air.Fixed orifice, cap tube, orCondenser designed to be 30° F. warmer piston for metering device.than outdoor air passing over it.Design Operating Refrigerant in evaporator will boil at 40° F.Conditions:(75° indoor air − 35° design temp Indoor air: 75° F.difference = 40° F. Saturation Relative Humidity: 50%.Temperature).Outdoor air: 95° F.Refrigerant in condenser will condense at*Note: Always refer to 125° F.the manufacturer (95° outdoor air + 30° design temp specifications if possible.difference = 125° F. Saturation temperature).Evaporator airflow = nominal 400 CFM/ton.Measured superheat should = 8-10° F.Measured sub-cooling should = 6-8° F.Suction pressure should = 68.5 PSIG (+/−2PSIG).High side pressure should = 278 PSIG (+/−2 PSIG).Suction line temperature should be 40° F.saturation + 8-10° F. superheat = 48-50° F.Liquid line temperature should be 125° F.saturation − 6-8° F. sub-cooling = 119-117° F.
10-12 SEER Equipment (R-22 Refrigerant)System Characteristics:Nominal Operating Parameters:Standard size evaporator.Evaporator designed to be 35° F. colder Larger size condenser.than return air.Metering Device = Thermal Condenser designed to be 25° F. warmer or Thermostatic Expansion than outdoor air passing over it.Valve (TXV).Refrigerant in evaporator will boil at 40° F.Design Operating (75° indoor air − design temp difference = Conditions:40° F. Saturation Temperature).Indoor air: 75° F.Refrigerant in condenser will condense atRelative Humidity: 50%.120° F.Outdoor air: 95° F.(95° outdoor air + 25° design temp *Note: Always refer to thedifference = 120° F. Saturation manufacturer specifications temperature).if possible.Evaporator airflow = nominal 400 CFM/ton.Measured superheat should = 8-10° F.Measured sub-cooling should = 6-8° F.Suction pressure should = 68.5 PSIG (+/−2 PSIG).High side pressure should = 259.9 PSIG (+/−2 PSIG).**Suction line temperature should be 40° F.saturation + 8-10° F. superheat = 48-50° F.Liquid line temperature should be 120° F.saturation − 6-8° F. sub-cooling = 114-112° F.**The lower discharge pressure versus standard efficiency equipment provides a smaller pressure difference across the compressor, and requires less energy to operate making the system more efficient. The higher efficiency comes at the cost of poor operation when operated in low ambient conditions. Some manufacturers have incorporated a two-speed condenser fan to rectify this problem. Even so a two speed motor and the control to operate it cost more up front. The efficiency upgrade will pay for itself.
12-20+ SEER Equipment (R-22 Refrigerant)System Characteristics:Nominal Operating Parameters:Larger size evaporator.Evaporator designed to be 30° F. colder Larger size condenser.than return air.Metering Device = Condenser designed to be 20° F. warmer Thermal Expansion than outdoor air passing over it.Valve (TXV).Refrigerant in evaporator will boil at 45° F.Design Operating (75° indoor air − 30° design temp Conditions:difference = 45° F. Saturation Indoor air: 75° F.Temperature).Relative Humidity: 50%.Refrigerant in condenser will condense atOutdoor air: 95° F.115° F.*Note: Always refer to (95° outdoor air + 20° design temp the manufacturer difference = 115° F. Saturation temperature)specifications if possible.Evaporator airflow = nominal 400 CFM/ton.Measured superheat should = 8-10° F.Measured sub-cooling should = 6-8° F.Suction pressure should = 76 PSIG (+/−2PSIG).High side pressure should = 243 PSIG (+/−2 PSIG).**Suction line temperature should be 45° F.saturation + 8-10° F. superheat = 53-55° F.Liquid line temperature should be 115° F.saturation − 6-8° F. sub-cooling = 109-107° F.**The lower discharge in combination with high suction pressure versus standard and high efficiency equipment provides a smaller pressure difference across the compressor, and requires less energy to operate making the system more efficient. The higher operating efficiency comes at the cost of lower latent heat capability, so this system may not dehumidify as well. It will also incorporate some of same the controls that the high efficiency equipment will incorporate.
10-12 SEER Equipment (R-410a Refrigerant)*System Characteristics:Nominal Operating Parameters:Standard size evaporator.Evaporator designed to be 35° F. colder Larger size condenser.than return air.Metering Device = Condenser designed to be 25° F. warmer Thermal Expansion than outdoor air passing over it.Valve (TXV).Refrigerant in evaporator will boil at 40° F.Design Operating (75° indoor air-35° design temp difference = Conditions:40° F. Saturation Temperature).Indoor air: 75° F.Refrigerant in condenser will condense atRelative Humidity: 50%.120° F.Outdoor air: 95° F.(95° outdoor air + 25° design temp *Note: Always refer to difference = 120° F. Saturation temperature)the manufacturer Evaporator airflow = nominal 400 CFM/ton.specifications if Measured superheat should = 8-10° F.possible.Measured sub-cooling should = 6-8° F.Suction pressure should = 118.9 PSIG (+/−2 PSIG).High side pressure should = 416.4 PSIG (+/−2 PSIG).**Suction line temperature should be 40° F.saturation + 8-10° F. superheat = 48-50° F.Liquid line temperature should be 120° F.saturation − 6-8° F. sub-cooling = 114-112° F.*It should be noted: As far as operating conditions are concerned, the only difference in operation between R-22 unit and R-410a units is the operating pressures.**The lower discharge pressure provides a smaller pressure difference across the compressor, and requires less energy to operate making the system more efficient. The higher efficiency comes at the cost of poor operation when operated in low ambient conditions. Some manufacturers have incorporated a two-speed condenser fan to rectify this problem. Even so a two speed motor and the control to operate it cost more up front. The efficiency upgrade will pay for itself.
12-20+ SEER Equipment (R-410a Refrigerant)*System Characteristics:Nominal Operating Parameters:Larger size evaporator.Evaporator designed to be 30° F. colder Larger size condenser.than return air.Metering Device = Condenser designed to be 20° F. warmer Thermal Expansion than outdoor air passing over it.Valve (TXV).Refrigerant in evaporator will boil at 45° F.Design Operating (75° indoor air − 30° design temp Conditions:difference = 45° F. Saturation Temperature).Indoor air: 75° F.Refrigerant in condenser will condense atRelative Humidity: 50%.115° F.Outdoor air: 95° F.(95° outdoor air + 20° design temp *Note: Always refer to difference = 115° F. Saturation temperature)the manufacturer Evaporator airflow = nominal 400 CFM/ton.specifications ifMeasured superheat should = 8-10° F.possible.Measured sub-cooling should = 6-8° F.Suction pressure should = 130.7 PSIG (+/−2 PSIG).High side pressure should = 389.6 PSIG (+/−2 PSIG).**Suction line temperature should be 45° F.saturation + 8-10° F. superheat = 53-55° F.Liquid line temperature should be 115° F.saturation − 6-8° F. sub-cooling = 109-107° F.*It should be noted: As far as operating conditions are concerned, the only difference in operation between R-22 unit and R-410a units is the operating pressures.**The lower discharge in combination with high suction pressure provides a smaller pressure difference across the compressor, and requires less energy to operate making the system more efficient. The higher operating efficiency comes at the cost of lower latent heat capability, this system may not dehumidify as well. It will also incorporate some of same the controls that the high efficiency equipment will incorporate.
When charging a refrigeration system, the following steps should be followed:                1. Inspect filters, evaporator coils, condensers coils and blower for dirt and clean if needed. If condenser is washed, let it dry before charging.        2. Make sure evaporator airflow is correct. (Nominal 400 CFM/Ton for A/C (350 CFM/ton in humid areas) 450 CFM/ton for Heat pumps)        3. Determine type of refrigerant.        4. Determine type of metering device.        5. Measure indoor/outdoor ambient air conditions (wet bulb and dry bulb).        6. Determine proper superheat or subcooling. (Use Manufacturer's chart if available.)        7. Attach Refrigeration System Analyzer (RSA) to service valve parts.        8. Attach temperature probe (to suction line for superheat measurement, to liquid line for subcooling measurement).        9. Verify refrigerant selection in manifold.        10. Determine the charging requirements        Charge directly by superheat or subcooling.        Note: Watch pressures while charging by superheat and subcooling methods to assure system is operating properly. Always check evaporator and total superheat on TXV systems to assure correct TXV operation.        11. Verify system pressures and saturation temperatures are within manufacturer's design criteria.        
Deviation from the correct charge will have a negative impact on the performance or operation of the refrigeration system. Systems utilizing a fixed metering device without any other mechanical problems and proper airflow and load will exhibit the following symptoms if improperly charged to a low charge (undercharge):                Low suction pressure.        Low liquid pressure.        High total superheat.        Low compressor amps.        Poor system performance.        Coil may be freezing.        Possible overheating of compressor.        
Systems utilizing a fixed metering device without any other mechanical problems and proper airflow and load will exhibit the following symptoms if improperly charged to a high charge (overcharge):                High suction pressure.        High liquid pressure.        Low total superheat.        Possibly higher than normal compressor amps.        Poor system performance.        Lack of humidity control.        
Systems utilizing a TXV without any other mechanical problems and proper airflow and load will exhibit the following symptoms if improperly charged to a low charge (undercharge):                Evaporator superheat normal or high.        Low condenser subcooling.        Poor performance at full or partial load.        Possible overheating of compressor.        
Systems utilizing a TXV without any other mechanical problems and proper airflow and load will exhibit the following symptoms if improperly charged to a low charge (undercharge):                Evaporator superheat normal.        High liquid pressure.        High condenser subcooling.        Poor performance at full or partial load.        
Industry studies show that approximately 70% of residential air conditioning systems are operating with refrigerant charge and airflow problems. Unlike lab testing done under a single set of closely held conditions, charging an air conditioning system in the field by a technician is often a complicated and dynamic process due to nonstandard conditions and constantly changing load conditions that technicians typically encounter. As load conditions change or vary from standard conditions inside or outside (ambient conditions) the conditioned space, so do performance and operational targets. System pressures, saturation temperatures, superheat, subcooling, airflow latent sensible split, power consumption, and work output all vary as the load and or the power supply (voltage) increases or decreases. Installation factors like line set length, lift in suction line insulation, and duct design also affect performance. Additionally as a system is serviced (particularly as refrigerant is added or removed) the operational characteristics again vary as the system reaches a new point of equilibrium which again changes the capacity and the rate which the sensible and latent load is handled. Determining when this new state of equilibrium is reached is also a challenge that can lead to excessive wait times to complete service.
Due to a constantly moving target, and variables associated with the installation often not accounted for in the field, acquisition and management of the data used to resolve the target performance indicators must also be as dynamic as the system itself to more accurately evaluate the performance of the system in field practice. Managing all of the data independently and manually requires the technician to carefully and quickly gather the measurement data, use several look up tables, and make manual calculations which can result in many errors from simple transcription to that of calculation or even resulting change in load conditions faster than the data can be hand obtained. Additionally, readings and calculations are not humanly possible in real time; and the variables are changing in real time presenting, at best, a fuzzy picture of the operational performance. These problems are amplified under low load and during periods of low ambient conditions due to system characteristics and the short amount of time that the system operates to satisfy the load requirements. Manual calculation is less accurate and subject to more error and cumbersome techniques making it often impractical to do in many field installations.